System and method for non-destructive decontamination of sensitive electronics using soft X-ray radiation

ABSTRACT

A method is provided for decontaminating biological pathogens residing in an enclosure of an electronic device. The method includes: identifying materials used to encase the enclosure of the electronic device; tailoring x-ray radiation to penetrate the materials encasing the enclosure; and directing x-ray radiation having a diffused radiation angle towards the electronic device.

FIELD

The present disclosure relates generally to decontamination ofbiological hazards and, more particularly, to a system and method fornon-destructive decontamination of sensitive electronic equipment.

BACKGROUND

When military personnel conduct missions in contaminated environments,there is an eminent need for a decontamination system for the electronicequipment used to support the missions. The ability to maintain materialintegrity of sensitive electronic devices is a key attribute of anydecontamination system. This is particularly true in view of the highcost associated with such electronic devices. In addition, thedecontamination system should be transportable with minimal impact tothe mission.

Radiation sterilization is generally much less disturbing than usingeither reactive oxidizers like chlorine or high temperature autoclaving.For instance, quartz-jacketed mercury lamps emitting 254 nm ultravioletlight are effective surface sterilizers, but unfortunately the lightcannot penetrate even a single sheet of paper. In contrast,decontamination by 10 MeV electron beams used by the U.S. PostalService, causes significant damage to the target and requires expensiveand cumbersome fixed infrastructure (facilities, power, and shielding).

Soft x-ray radiation offers an efficient, non-destructive, cold,chemical-free sterilization method. However, there is a need to tailorthis approach for decontamination of electronic equipment. Thestatements in this section merely provide background information relatedto the present disclosure and may not constitute prior art.

SUMMARY

A method is provided for decontaminating biological pathogens residingin an enclosure of an electronic device. The method includes:identifying materials used to encase the enclosure of the electronicdevice; tailoring x-ray radiation to penetrate the materials encasingthe enclosure; and directing x-ray radiation having a diffused radiationangle towards the electronic device.

Further areas of applicability will become apparent from the descriptionprovided herein. It should be understood that the description andspecific examples are intended for purposes of illustration only and arenot intended to limit the scope of the present disclosure.

DRAWINGS

FIG. 1 is a flowchart illustrating an exemplary decontaminationtechnique for electronic equipment;

FIG. 2 is a graph illustrating how x-ray radiation having differentphoton energy levels penetrates polypropylene plastic;

FIG. 3 is a graph illustrating kill times for an exemplary biologicalpathogen;

FIG. 4 is a graph illustrating the interaction strength of x-rayradiation with an embedded spore in a plastic environment;

FIG. 5 is a diagram depicting a conventional x-ray source;

FIG. 6 is a diagram depicting an x-ray source that has been modified todiffuse the radiation; and

FIG. 7 is a diagram of an exemplary decontamination system; and

FIG. 8 is a diagram illustrating a decontamination system equipped withmultiple types of x-ray heads.

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

FIG. 1 illustrates a rapid and non-destructive decontamination techniquefor electronic equipment. When exposed to a contaminated environment,biological pathogens may penetrate the exterior surface of an exposedpiece of electronic equipment. In this case, x-ray radiation may be usedto sterilize biological pathogens found in interior compartments of theequipment. It is envisioned that x-ray radiation may be used tosterilize other type of decontaminates which may reside within a pieceof electronic equipment.

First, the materials which comprise those parts of the contaminatedequipment between its exterior surface and the deepest internalcontamination site, and their thicknesses and densities, must bedetermined as shown at 12. X-ray radiation can then be tailored at 14 topenetrate those materials of the exterior surface of the equipment.X-ray radiation of different photon energies penetrates differentmaterials to different depths. The x-ray transmission, T_(i), ofmaterial I used to construct a piece of equipment is given byT_(i)=e^(−σ) ^(i) ^(n) ^(i) ^(L) ^(i) ,where σ_(i) is the absorption material's atomic cross section, n_(i) isthe number density (atoms per cubic centimeter), and L_(i) is the pathlength that the x-rays follow through the absorption material. For acombination of several layers of different materials, the totaltransmission is

$T = {{\prod\limits_{i}T_{i}} = {\mathbb{e}}^{- {\sum\limits_{i}{\sigma_{i}n_{i}L_{i}}}}}$Each material's atomic cross section is a function of the photon energy.Above the K-shell binding energy, the cross section varies as theinverse square of the photon energy. This strong relationship results ina wide range of transmission T versus energy. An energy level for thex-ray radiation is preferably chosen at which T=e⁻¹.

The ideal x-ray photon energy penetrates exactly through the materialcontaining a contaminant, but no more. Use of high energy radiation iswasteful because a preponderance of the incident energy passes throughthe target without significant energy deposition. On the other hand,very soft x-rays are absorbed by short depths of a material and thus donot penetrate to the location of embedded contaminants. Thus, it ispreferable to select the lowest photon energy level needed to passthrough the exterior surface of the electronic equipment. For differenttypes of electronic devices, there will be a relatively narrow range ofenergies which is best suited, matched to the devices mean absorptiondepth.

FIG. 2 illustrates an x-ray photon transmission curve for typicalplastics (i.e., 2.5 mm of polypropylene plastic). At 5 keV, only a fewpercent of the radiation penetrates the plastic such that bacteria onthe other side of the plastic may survive. At 12 keV, most radiationpasses through the plastic without interacting with the bacteria.However, at 8 keV, the radiation effectively penetrates the plastic tokill any embedded bacteria. Therefore, x-ray radiation having a photonenergy of 8 keV is preferable for electronic equipment having a plasticexterior surface. For comparison, it has been determined that radiationhaving 22 keV effectively penetrates one millimeter of aluminum. It isnoteworthy that these energy levels are far above the 1.8 keV at whichsilicon absorbs and thus should not affect the semiconductor componentswhich comprise the equipment. However, the energy levels are low enoughthat chip packaging will provide some shielding.

Since most electronic devices have varied constituents, it may be moreadvantageous to use a source spectrum with several sharp peaks. Forexample, a source may have two peaks in the spectrum—one that penetratesplastic and a second one that penetrates aluminum. This may be achievedwith an anode made of an alloy, such as copper-silver or copper-cadmium,or alternatively a patterned plating of higher Z metal on a copperanode. Broad spectrum irradiation like Bremsstrahlung, while alwaysaccompanying line radiation to some extent, is inefficient fordecontamination because the substantial low-energy fraction will notpenetrate the target while the high energy tail will pass through and belost. Compton scattering is mostly negligible at these low energies. Insilicon at 8 keV, the photoelectric cross section is almost three ordersof magnitude higher than Compton. At 22 keV in carbon, the two crosssections are comparable and will be discussed in relation to thepathogen kill mechanism below.

When the biological pathogen residing in the equipment is known, thex-ray radiation may be further tailored to sterilize or kill the hazard.For instance, the dose of radiation (i.e., the duration of radiation)applied to the equipment is also determined. The practicality of thisconcept was demonstrated with a feasibility experiment. Samples of 10⁶spores of Bacillus subtilis, which is a non-hazardous surrogate forBacillus anthracis, were first placed in a test environment and exposedto a dose of x-ray radiation from a copper anode source having photonenergies primarily around 8 keV. Irradiated and control samples werethen individually incubated in soy broth at 35° C. for a week. Sampleswith one or more viable spores produce a cloudy infusion, while acompletely sterilized sample remains clear. At delivered doses of over1.5 J/cm², all samples were completely sterilized. The highest dosedelivered to a sample that remained incompletely sterilized was 0.117J/cm². Hence the 8 keV x-ray kill dose for 10⁶ spores of our surrogatefell somewhere between those two values. FIG. 3 illustrates theirradiation time required for a complete kill of 10⁶ spores as afunction of input electrical power for the upper and lower kill dosebounds. It is well established that killing spores is the mostchallenging sterilization problem. The radiation dose sufficient to killbacterial spores is much higher than that required to kill hydratedactive bacteria and other biological pathogens. Accordingly, radiationdoses for active bacteria and other biological pathogens can beempirically derived in a similar manner.

Any radiation that is energetic enough to penetrate centimeters ofcontaminated environment will necessarily have a low inelastic crosssection with an individual spore. Given that, the lower the photonenergy, the more likely an interaction with a spore will occur. In fact,the combination of the x-ray requirements of penetrating the spore'ssurrounding and also being absorbed by the spore results in a band passcurve as shown in FIG. 4. Note the peak of the curve is near thelow-energy cut off determined by the contaminated environment x-raytransmission function.

Moreover, the electron produced by a soft x-ray absorption event isideally suited to deliver a maximum energy transfer to the spore. Abacterial spore (properly referred to as “endospore”) is a dormant formthat certain bacteria develop when confronted with difficultenvironmental conditions. It is characterized by a significant waterloss (down to 20% or less), concentration of minerals (particularlycalcium), formation of a multiple membrane outer coat and effectivelyceasing metabolism. When a soft x-ray is absorbed in an endospore, afast-moving primary photoelectron and a slow recoiling ion are produced.The photoelectron traverses the body of the endospore causing secondaryionizations and producing secondary electrons that travel along theirpaths. The result is a ballistic trajectory of multiple chargedisplacements. This damage trail can be lethal to the endospore if itsignificantly disrupts certain structures such as membranes or criticalmolecules like DNA. Reactive chemistry can also take place along theionization trajectory because of all the ions and free radicalsproduced.

For an 8 keV primary photoelectron, the mean free path in protein isvery close to 1 μm, or is almost exactly matched to the size of theendospore. At higher energies, the primary photoelectron will exit theendospore long before depositing its full energy. For instance, at 20keV, the mean path is around 9 μm. Electrons produced by Comptonscattering have the same problem, as Compton is a higher energy process.

Design of the x-ray source for decontamination applications isqualitatively different than for conventional x-ray tubes used forimaging. Importantly, the x-ray emitting area needs to be large so thatsharp shadows in the illuminated volume are avoided. If sharp, highcontrast shadows occur, microscopic pathogens could escape from theirradiation and circumvent the desired sterilization. Since x-rays areemitted from the outermost few microns of anode material which receiveselectron bombardment, the electron beam must be diverged and spreadevenly to impinge over the full surface of the anode to achieve thelargest effective source size. To this end, the electric field guidingthe electrons must be crafted to diverge from the cathode and intersectthe anode uniformly, to the greatest extent possible. This technique ofmanipulating the electric field distribution in the x-ray source isreferred to herein as “field sculpting”.

Traditional x-ray sources used for imaging applications are designed aspoint-source emitters as shown in FIG. 5. Briefly, the x-ray source 30is comprised of a cathode 31 and an anode 32 housed in an electricallyconducting, grounded vacuum enclosure 33. The cathode 31 is electricallycoupled via a load resistor 35 to a power supply 36. In operation, thecathode emits electrons when energized by the power supply 36. Emittedelectrons (paths indicated by dotted lines 37) follow the electricfields and are accelerated towards the anode 32 which in turn emitsx-ray radiation 38 (indicated by dashed lines) when the electronsimpinge upon its surface. The cathode acquires a voltage (called theself-bias voltage) equal to the product of the load resistance and theemitted electron current. The combination of the cathode's acquirednegative voltage, the enclosure ground, and the anode's positive highvoltage forms a three-element electron lens, which focuses the electroncurrent density to a small point. All x-ray radiation is generated atthat point. Although desirable in imaging applications, this sourceconfiguration produces sharp shadows of absorbing materials 39 (which inapplication would be objects in the contaminated environment such assemiconductor devices, electric leads or wires, for example) asindicated by the plot of intensity versus position behind the absorber.This may obscure the biological hazards and dramatically reducedecontamination efficacy.

To make a diffuse x-ray lamp, it is necessary for a large area of theanode surface to emit x-rays. This requires the electron current to bespread wide, avoiding focusing effects. A modified x-ray source designis shown in FIG. 6. Three major modifications have been made to theclassical design to accomplish this electron spreading. First, thecathode 41 is electrically tied to ground to avoid any self-biasvoltage; the load resistor has been removed. Second, the surface figureof the anode 42 has been curved into a concave shape. Third, asupplementary electrode called the field sculpting electrode 43 isplaced surrounding the electron current in close vicinity to the cathodeand is biased by a variable voltage 44. Although any one of thesechanges produces a partial result, the combination of these threechanges causes the electric field lines to spread out, drawing theelectron current 45 to impact uniformly across the anode surface. Inturn, this results in an illumination of the absorber 46 which isdiffuse, as indicated by the x-ray trajectories 47. The term “diffusedradiation angle” refers to the source possessing the characteristic of alarge radiating surface area as viewed by the absorbing material in thecontaminated environment, resulting in lowered shadow contrast to avoidhaving local unirradiated regions. The resulting x-ray intensity patternbehind the absorber does not fall to zero, meaning even if pathogenswere to reside behind the absorber they would still be irradiated. Thediffused radiation angle may be quantified by a measure analogous to afocal ratio or F-number of a camera. For example, the diffused radiationangle may be measured by an “F-number” defined as the distance from thesource to the object being irradiated divided by the size of the x-rayspot. For most conventional x-ray sources, the source size is around 100microns or smaller, such that its “F-number” is around 10,000. Thediffused radiation angle employed by this disclosure gives an “F-number”less than 10 with a final design goal of less than four.

Additionally, this x-ray source may be configured to irradiate over avery wide angle by positioning the output window as close as possible tothe anode. X-rays are generated in the first few micrometers of theanode surface that is bombarded with electron current. Any location inthe irradiated zone in a clear line of sight to the active anode surfacewill receive x-rays. The design and location of the output window can beconfigured to transmit close to a full 27i steradians of irradiatedsolid angle.

Furthermore, the radiation should thoroughly penetrate the materialscovering, surrounding or otherwise obstructing the biological hazard.The x-ray radiation should not pass through the contaminated materialshaving failed to interact with the biological hazard. High energy x-rayphotons will penetrate denser materials, but the resultant scatteringcross-section of the photon is low. Therefore, a larger flux of x-rayphotons is required, leading to longer exposure times to achieve asufficient kill dose. This is the reason it is advantageous to choosethe x-ray photon energy consistent with the materials needing to bedecontaminated.

The photon energies produced by an x-ray source can be scaled throughthe judicious choice of the anode materials. This is understood throughMoseley's empirical formula for k-alpha x-rays. The formula shows thex-ray photon energy is dependent on the square of the atomic number ofan elementE_(K) α (Z−1)²where E_(K) is the x-ray photon energy and Z is the atomic number of theanode material. For instance, an x-ray source having a molybdenum (Z=42)anode will generate radiation having a photon energy of 18 keV. Incomparison, a silver (Z=47) anode can generate radiation having a photonenergy of 22 keV. It is envisioned that x-ray sources will be fabricatedwith different anode materials to ensure penetration through variousmaterial compositions providing decontamination radiation inside theelectronic device. It is also understood that an x-ray source may employdifferent types of cathodes, including but not limited to thermionicemitters, such as tungsten-thorium alloy, tantalum, and others, as wellas cold cathodes which could be metallic wires or exotic materials likecarbon nanotubes.

FIG. 7 illustrates an exemplary portable, cart-like decontaminationsystem which may be used to deploy this technology. The decontaminationsystem is comprised of a radiation chamber and one or more x-ray headsarranged to radiate the chamber. Each of the x-ray heads are configuredto generate x-ray radiation having a diffused radiation angle in themanner described above. The x-ray head will be made more compact by theuse of ultra-high dielectric strength insulators, and weight will bereduced. The vacuum seal will be made permanent. The beryllium windowwill be shuttered for safety, and interlocks will be installed toprevent operation without radiation shielding.

With reference to FIG. 8, the decontamination system is preferablyequipped with multiple x-ray heads. In one exemplary embodiment,different x-ray heads are oriented at different angles within thechamber. In this way, different x-ray heads may be selected to generatex-ray radiation depending upon the object being decontaminated. Forexample, each of the x-ray heads may employ a copper anode suitable forpenetrating plastic materials, but only one of the exterior surfaces ofthe object is made of plastic. In this example, the x-ray head orientedtowards the plastic exterior surface is used to penetrate the object.

In another exemplary embodiment, different x-ray heads may be configuredto generate x-ray radiation at different photon energy levels. Forinstance, one x-ray head may employ a copper anode while another x-rayhead employs a silver anode. Thus, different x-ray heads may be useddepending on upon the material of the object to be decontaminated.Likewise, different x-ray heads may be used to penetrate differentenclosures of the same object, where the different enclosures may beencased by different materials.

X-ray radiation may also be used for decontaminating the exteriorsurface of electronic equipment. To do so, the portable decontaminationsystem may be equipped with one set of x-ray heads for producing lowerenergy x-ray radiation (e.g., 8 keV) and another set of x-ray heads forproducing high energy x-ray radiation (e.g., 15-30 keV). Lower energyx-rays have larger scattering cross-sections and hence interact stronglywith biological pathogens found on an exterior surface of any object. Onthe other hand, higher energy x-rays are needed to penetrate theexterior surface of the object. Penetrating x-rays may interact withbiological pathogens within an enclosure of an object by producingfluorescence. Although the conversion efficiency is low, these photonshave scattering cross-sections 900 times larger, thereby achievingeffective decontamination within a cavity.

In an alternative configuration, the decontamination system may beequipped with ultraviolet radiation sources for effectuating surfacedecontamination. Conventional ultraviolet lamps are readily available inthe marketplace. Ultraviolet radiation has proven effective fordecontaminating and sterilizing biological pathogens. For example, killdoses for UV radiation at 254 nm has been measured. For Bacillusanthracis, doses delivered at 45 mJ/cm² achieved a 99.9% kill rate ofthe pathogen on the surface. Doses are low because every photon inabsorbed. However, ultraviolet radiation does not penetrate materials.Therefore, x-ray heads are also employed in the manner described abovefor internal decontamination.

The above description is merely exemplary in nature and is not intendedto limit the present disclosure, application, or uses.

1. A method for decontaminating biological pathogens residing in anenclosure of an electronic device, comprising: identifying materialsused to encase the enclosure of the electronic device; tailoring x-rayradiation to penetrate the materials encasing the enclosure; anddirecting x-ray radiation having a diffused radiation angle towards theelectronic device.
 2. The method of claim 1, wherein tailoring x-rayradiation further comprises determining a photon energy level for thex-ray radiation needed to penetrate the materials encasing the enclosureof the electronic device.
 3. The method of claim 2, further comprisesgenerating x-ray radiation having a photon energy of approximately 8 keVwhen the material encasing the enclosure is plastic.
 4. The method ofclaim 2, further comprises generating x-ray radiation having a photonenergy of approximately 22 keV when the material encasing the enclosureis aluminum.
 5. The method of claim 1, wherein tailoring x-ray radiationfurther comprises selecting the lowest energy level needed to passthrough the materials encasing the enclosure of the electronic device.6. The method of claim 1, wherein tailoring x-ray radiation furthercomprises determining a dose of x-ray radiation needed to kill asuspected biological pathogen residing in the electronic device.
 7. Themethod of claim 1, further comprises generating x-ray radiation having adiffused radiation angle by accelerating electrons from a cathodetowards a concave surface of an anode.
 8. The method of claim 1 furthercomprises generating x-ray radiation having a diffused radiation angleby electrically grounding a cathode to minimize self-bias voltage. 9.The method of claim 1 further comprises generating x-ray radiationhaving a diffused radiation angle by disposing a secondary electrodeproximate to a cathode for shaping the x-ray radiation.
 10. A method fordecontaminating biological pathogens associated with an electronicdevice, comprising: identifying materials used to encase an enclosurewithin the electronic device; directing x-ray radiation having a firstphoton energy level towards the electronic device; and directing x-rayradiation having a second photon energy level towards the electronicdevice.
 11. The method of claim 10 further comprises directing x-rayradiation having a diffused radiation angle towards the electronicdevice.
 12. The method of claim 10 further comprises tailoring the firstphoton energy level to penetrate an exterior surface of the electronicdevice; and tailoring the second photon energy level to decontaminatethe exterior surface of the electronic device.
 13. The method of claim10 further comprises tailoring the second photon energy level to have alower energy level than the first photon energy level.
 14. The method ofclaim 10 further comprises tailoring the first photon energy level topenetrate an exterior surface of the electronic device; and tailoringthe second photon energy level to penetrate a different exterior surfaceof the electronic device which is comprised of a different material thansaid exterior surface.
 15. A method for decontaminating biologicalpathogens associated with an electronic device, comprising: directingultraviolet radiation towards an exterior surface of the electronicdevice; tailoring x-ray radiation to penetrate the exterior surface ofthe electronic device by identifying materials which comprised theexterior surgface and selecting a photon energy level for the x-rayradiation that penetrates the identified materials; and directing x-rayradiation having a diffused radiation angle towards the electronicdevice.